The present invention relates to anti-reflection coatings.
Anti-reflection coatings are applied to transparent substrates such as optical components with a view of reducing the reflection of light of a predetermined wave lengths at the substrate surface.
Usually it is desired to reduce reflection from the substrate at more than one wave length, for example over a band of wave lengths.
Among prior basic proposals are coatings comprising two and three layers; the two layer coating comprises outer and inner layers having thicknesses of .lambda.o/4 and .lambda.o/2 respectively, where .lambda.o is a wave length lying within a band-width over which reflection is to be reduced, while the three layer coating has layers of thickness .lambda.o/4, .lambda.o/2, .lambda.o/4 respectively.
These coatings are satisfactory to meet theoretical anticipation and requirements.
However, it has not been possible to obtain anti-reflection coatings just same as the theoretical anticipations in industrial processes. Thus, anti-reflection performance is greatly reduced due to the difference between actually manufactured coating and those theoretically anticipated.
It was found that the difference was mainly brought about by heat utilized in the coating process. In the coating process, it is required to heat a substrate to a certain high temperature in order to increase mechanical strength of the coatings. When so heated, it is observed that the coating is made inhomogeneous in refractive index. This phenomenon is conspicuous when layers are thick. For example, in a case of a zirconium oxide layer which is popularly used as a substance for a thick and high-refractive index layer, refractive indeces of the layer decreases from a substrate side to an air side as the thickness of the layer increases. It is also observed that when the 1/2.lambda.o layer of the two layer or three layer anti-reflection coating was inhomogeneous in refractive index, reflection from the optical component was greatly elevated, so that anti-reflection performance was unsatisfactory for practical use.
Such defect is difficult to overcome by controlling of vacuum evaporation and hence satisfactory anti-reflection coatings have not been obtained.